In this chapter, we discuss a completely different approach to pattern recognition, a methodology based on an analogy to language understanding. These methods have not often been used recently, primarily because they are very sensitive to noise and distortion. However, for certain applications, they may be appropriate, and the student is advised to learn enough about this topic to recognize potential applications.

Consider a boundary segment represented by a chain code. Each step in that chain code is a symbol, an integer between 0 and 7, so that the boundary segment is represented by a string of symbols: What makes syntactic methods work is the analogy between this string of symbols and the string of symbols which show up in the description of a formal language.

Terminology

To make more progress in this area, we need to define some terminology. The definitions are in reference to analysis of strings of symbols, such as occur in language analysis.

A terminal symbol: A word, like “horse,” “aardvark,” “professor,” “runs,” “grades.” Terminal symbols may also be line segments, parts of a picture, or other features. Generally, we denote terminal symbols using lower case. Most often, terminal symbols are denoted by a single symbol, like “a” or “0” but in the example of words from English, the terminal symbols are words, not letters.